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EHT 2014

The Event Horizon Telescope is the first astronomical instrument capable of imaging the horizon of a known black hole. By assembling a global network of existing millimetre and sub-millimetre wavelength observatories, the EHT can access the extraordinary resolutions required via Very Long Baseline Interferometry. Already it has detected horizon scale structure around the supermassive black holes at the centre of the Milky Way and the giant elliptical galaxy M87.

This is the second in a conference series designed to bring together the full EHT community, from instrument builders to theoretical modellers, for the purpose of fully exploiting the unique opportunities that the EHT provides. The goals of this conference will be:

Describe the broad scientific context of astrophysical black holes and its relationship to EHT observations, including how the EHT can provide insight into systems ranging from pulsars to galaxy clusters by probing accretion and energetic outflows on the scale of the Schwarzschild radius.

Discuss developments in our understanding of the astrophysics of the initial EHTtargets: the supermassive black holes at the centre of the Milky Way (Sgr A*) and M87.

Present new EHT-driven results.

Describe the current technical status and discuss the planned development of the EHT, including the imminent inclusion of the Atacama Large Millimeter/submillimeter Array and the South Pole Telescope.

Event-horizon-scale structure of M87 in the middle of the VHE enhancement in 2012

The radio galaxy M87 is an excellent laboratory for investigating the formation process of the relativistic jet and the production mechanism of high energy particles and photons in the vicinity of super-massive black holes. VLBI observations at 1.3 mm can address at least two issues concerning the fundamental nature of M87. The first is the event-horizon-scale structure of the jet launching region, and another one is the production mechanism of very high energy (VHE; > 100 GeV) photons at there. In this talk, I report on new 1.3 mm VLBI observations of M87 with the EHT during the VHE enhancement in 2012. These observations provide the first measurements of closure phase, imposing new constraints on accretion-disk/jet models for M87, and also the first constraints on the innermost structure of the relativistic jet on scales of a few Rs during VHE variability. I discuss results and their implications for jet/disk models and also VHE models for M87, as well as future prospects of EHT observations of M87.

Mass accretion rate on the SMBHs is one of the fundamental parameters used to investigate AGNs. Faraday Rotation Measure (RM) observations at mm/sub-mm wavelengths is one of the powerful methods to derive the mass accretion rate of hot accretion flows towards our galactic center, Sgr A* (e.g., Marrone et al. 2006). Based on this scheme, we conducted an SMA observation to apply this method to M 87, which is one of the primary target for our submm VLBI observations, in February 2013. We succeeded to derive an RM of (2.1 ± 1.8) × 10^5 rad m^-2, it gives the range of the mass accretion rate (M_dot) between 0 and 9.2 × 10^-4 M_sun yr^-1 at the distance of 21 rs from the SMBH. Our estimated M.is already two orders of magnitude smaller than the M_dot at the outer part of the accretion flow (~10^5 r_s) of 0.1 M_sun yr^-1 determined by X-ray observations (Di Matteo et al. 2003). This significant suppression of the M_dot at the inner region is expected with the radiatively inefficient accretion flow (RIAF) model. With future submm VLBI polarimetry towards jetted sources including M 87, we will derive the profile of accretion flow along the jet. It is very important itself for the study of the accretion process onto the SMBH, but also provide fundamental properties to derive BH parameters from the BH shadow imaging.

Geoff Bower, Academia Sinica Institute of Astronomy and Astrophysics

The Size and Morphology of Sgr A* at 7mm

Long wavelength measurements provide sensitive probes of the intrinsic structure of Sgr A* and of the scattering properties of the line-of-sight interstellar medium. At this wavelength, scattering dominates the apparent size of the source but careful closure amplitude techniques can provide highly accurate structural information. We present new results from the VLBA at 7mm wavelength that for the first time reveal two-dimensional intrinsic structure while also demonstrating the stability of the intrinsic size during periods of significant activity at NIR and X-ray wavelengths. These results also demonstrate the stability of the scattering medium over time. New observations of the Galactic Center pulsar PSR J1745-2900 show that the scattering properties of Sgr A* are spatially coherent over an angular scale of at least a few arc seconds. Analysis of the angular and temporal broadening data for the pulsar place the scattering medium at a distance of kiloparsecs away from the Galactic Center, resolving a significant mystery regarding the scattering medium.

Avery Broderick, Perimeter Institute & University of Waterloo

EHT Constraints on Jet Launching in M87

Because of the existence of a prominent jet, M87 provides an ideal source with which the EHT can critically test current jet launching paradigms. The EHT has already placed weak limits on the black hole spin based on generic arguments regarding limits on the image size. However, careful modeling of the jet structure can produce much more stringent constraints on both the black hole properties and the mechanisms responsible for the generation of the observed radio emission. I will describe how this modeling is performed, what the remaining chief uncertainties are and why we believe they can be controlled, and how existing EHT observations of M87 support models of electromagnetically launched jets powered by black hole spin.

Andrew Chael, Harvard-Smithsonian Center for Astrophysics

Probing Dynamical Activity near the Event Horizon with the EHT

Sgr A* regularly flares in the X-ray and near-IR on ~hour timescales, and the EHT has already detected interday variability in 1.3 mm emission on long and short baselines. The addition of highly sensitive long baselines in 2015 will allow for the resolution of time variable structure on sub-minute timescales. This opportunity to observe dynamical process on event horizon scales comes with the challenge of sparse visibility coverage, but several strategies can recover rich information from the limited samples. I will review sources of variability for the emission near supermassive black holes from minute to year timescales, and discuss the prospects of EHT observations for understanding event-horizon-scale dynamics.

Jim Cordes, Cornell University

Neutron Stars in the Galactic Center

I will discuss the prospects for finding additional radio pulsars in the Galactic center region and their utility for probing general relativity and other constituents in the region. This will include discussions of neutron star populations; radio wave scattering in and toward the Galactic center; issues and progress in discovering pulsars; and the precision to which discoverable pulsars can be timed.

Geoffrey Crew, MIT Haystack Observatory

EHT Correlation

Over the next few years the Event Horizon Telescope will greatly expand its capabilities from the current 8 Gbps at a few sites to 64 Gbps at a much larger number of sites. The good news is that this processing can proceed through 4 independent (16 Gbps) processing stages of 2 GHz of bandwidth. The bad news is that EHT stations come in multiple "flavors" each posing its own issues for correlation with DiFX (the current correlation option). This talk will discuss some of the issues and the road ahead.

Jason Dexter, University of California, Berkeley

Crescent Black Hole Images

Maximizing the science return on the Event Horizon Telescope project requires fitting models for spatially resolved black hole images to the data. These images can be calculated from accretion and jet theory, but theoretical uncertainties lead to systematic errors in the predicted images. In many cases, however, the images are dominated by the combined effects of Doppler beaming and light bending, leading to a characteristic “crescent” shape. I will discuss a geometric crescent model for black hole images based on these effects. The crescent outperforms other simple geometric shapes in the description of current data, can be used to simulate future EHT observations, and has implications for detecting a black hole shadow.

Shep Doeleman, MIT Haystack Observatory

Introduction to the EHT

The Event Horizon Telescope is a project to assemble a global Very Long Baseline Interferometric (VLBI) array that operates at mm and submm wavelengths. With stations ranging from Hawaii to the South Pole, the array will have the angular resolution to image the nearest super massive black holes, and the sensitivity to time resolve dynamics at the event horizon. The central goal is to spatially resolve a region of space time where gravity is dominant, and to detect strong field GR signatures. This introductory talk will outline the project history, enabling technical developments, planned observations, and excellent prospects for exciting new results in black hole studies.

Shep Doeleman, MIT Haystack Observatory

The EHT Wiki

The EHT Wiki is the primary vehicle for communication with the project. The site contains pages on science investigations, algorithmic development, new hardware, and staging information for observations and data processing. This talk will introduce the Wiki and walk through its various sections to show how it is used to help organize the EHT.

Heino Falcke, Radboud University Nijmegen

Jets and the radio emission from supermassive black holes

Vincent Fish, MIT Haystack Observatory

Detection and Variability of Closure Phases in Sgr A*

Closure phases measured on the Arizona-California-Hawaii triangle of the EHT over multiple years indicate that the 1.3 mm structure of Sgr A* is asymmetric on scales of a few Schwarzschild radii. The closure phase data provide new constraints on models of the quiescent emission from Sgr A*. Time variability in the closure provides evidence of structural changes on scales resolved by millimeter-wavelength VLBI. We discuss these results as well as other implications of the data.

Vincent Fish, MIT Haystack Observatory

Synthetic Data

You've just finished running your code, and you're certain that you (and only you) know exactly what the region around a supermassive black hole looks like. You could lie back and wait for the accolades to roll in, but why not take an extra moment to make testable predictions that even the observers can understand? Synthetic data can help.

Andrea Ghez, University of California, Division of Astronomy & Astrophysics

Stellar Orbits at the Galactic Center

Over the last 20 years, advances in high angular resolution imaging technology has enabled the motions of individual stars to be tracked at the Galactic Center.

This has provided the best evidence to date not only for the presence of a supermassive black hole at the center of our Galaxy, but for the existence of black holes in general. These high resolution measurements have also revealed an environment surrounding the black hole that is quite unexpected in a number of ways, challenging our understanding of the physical processes between black holes and their surround stars and gas. As the only galactic nucleus in which individual stellar orbits can be measured, the Galactic Center is now offering new insights into the fundamental physics of black holes, with unique tests of Einstein's theory of General Relativity on the horizon, and the astrophysics processes between black holes and their host galaxies that are thought to shape the co-evolution of central black holes and their host galaxies.

I will review the work done by the UCLA Galactic Center Group and others on measuring stellar orbits, with an emphasis on (1) our recent model for G2 as a binary star that has been driven to merge through three-body interactions with the central black hole, (2) precision measurements of the distance to the black hole (R0), which is a fundamental constant for many astrophysical studies (e.g., galactic structure), and the mass of the black hole (Mbh), which as a ratio with R0 (Mbh/R0) is a key ingredient for interpreting future EHT measurements of the black hole's shadow, and (3) upcoming tests of General Relativity that are within reach through precision measurements of stellar orbits.

GRAVITY is a new instrument combining the four 8m ESO Very Large Telescopes in Chile. Other than the BlackHoleCam / EHT with its focus on imaging the shadow of the black hole against the surrounding accretion flow, the goal of GRAVITY is to measure dynamical processes in the immediate vicinity of the black hole, for example the motion of matter close to the last stable orbit and relativistic effects in stellar orbits. Our presentation covers the experimental and astrophysical aspects of this project and highlights the complementarity with the submm interferometry to overcome the degeneracies in modelling the observations.

Gabriela Gonzalez, Louisiana State University

Advanced LIGO status and prospects to probe the strong gravity regime

Gravitational waves will allow scientists to test Einstein’s theory of General Relativity in the previously unexplored strong-field regime. Einstein’s theory of general relativity, as the most accepted theory of gravity, has been greatly constrained in the quasi-linear, quasi-stationary regime, where gravity is weak and velocities are small. Gravitational waves may carry information about highly dynamical and strong-field gravity that is required to generate measurable waves. Coalescing compact binaries are the most promising sources of gravitational waves accessible to ground-based interferometers, such as Advanced LIGO. Made of neutron stars and/or black holes that orbit each other hundreds of times a second just before they collide, the resulting waves are imprinted with information about the individual objects and the dynamical coalescence process. After reviewing the basic properties of gravitational waves, I will present an overview of the detector design and provide an update on the current status of Advanced LIGO and its ability to probe the strong gravity regime.

Kayhan Gultekin, University of Michigan

Stellar Dynamical Measurements of the Black Hole in M87 and Friends

Stellar dynamical measurements of black hole masses have become the de facto standard method. I will give a brief review of how this measurement method works, along with arguments for its overall reliability and caveats. Then I will turn my attention to the case of the black hole in M87. The black hole is undeniably large – billions of solar masses – but has a stellar dynamical mass measurement in disagreement with gas dynamical mass measurements at about the 2 sigma level. I will discuss potential systematic uncertainties in both measurements and avenues to reconciling the discrepancy.

Michael Hecht, MIT Haystack Observatory

Phasing ALMA

We are now two years into development of hardware and software for phasing ALMA, with emphasis on Band 6 (1.2 mm) but applicability to bands 3 and 7. Central to the effort is software to continuously calculate and apply a phasing solution, and the hardware interface cards (PIC) and other upgrades to the correlator that apply the solution, format the data and extract the correlated data stream. Upstream from the correlator is a hydrogen maser, which is the new ALMA time standard. Downstream from the correlator are four high speed Mark 6 data recorders collectively capable of capturing 64 Gb/s, and an optical fiber link to bring the signal from the AOS, where the antennas and correlator are located, to the OFS where the recorders are located. All the hardware is currently installed and functional. A formal acceptance review for the maser was held this past week, and acceptance of the rest of the hardware will be reviewed in December. While some debugging of software is ongoing, the basic software elements are in place and (more or less) functional. Formal commissioning activities are planned to begin in January.

Mareki Honma, National Astronomical Observatory of Japan

Super-resolution imaging with sparse modeling

The angular resolution is the most fundamental to imaging the event-horizon-scale structure of supermassive black holes, and in order to realize the highest angular resolution ever, the EHT has been developing a world-wide (sub)mm VLBI array under the international collaborations.

In order to boost the imaging capability of the EHT, we have been developing a new imaging method based on the technique so-called "sparse modeling", which allows us to directly solve the ill-posed Fourier-transform equations caused by incomplete sampling of visibilities.

We show that the images obtained based on such an approach indeed provide an angular resolution a few times better than that of the standard imaging. We report on our simulations and also preliminary results of its application to real data, and discuss its potential in imaging black hole shadows with the EHT.

Tim Johannsen, Perimeter Institute

Testing General Relativity with the EHT

In general relativity, astrophysical black holes are characterized uniquely in terms of their masses and spins and are described by the Kerr metric. The high sensitivity and resolution of the EHT will allow for unprecedented tests of the Kerr nature of black holes and, hence, of general relativity. I will present current and future limits on deviations from the Kerr metric from Sgr A* in the context of radiatively-inefficient accretion flow models. I will also show how largely model-independent constraints on such deviations can be obtained from the angular diameter of the photon ring of Sgr A* in combination with existing mass and distance measurements. Finally, I will describe a new general framework for the parametrization of deviations from the Kerr metric.

Michael Johnson, Harvard-Smithsonian Center for Astrophysics

Detection and Implications of Horizon-Scale Polarization in Sgr A*

The Event Horizon Telescope has measured compact emission in Sgr A* and M87 at resolutions comparable to their event horizons. Polarimetry with the EHT enables a powerful extension of this work, mapping magnetic field structures via the highly polarized synchrotron emission that is thought to dominate the compact emission. Sgr A* provides an especially attractive target for linear polarization studies with the EHT because it is unpolarized at the longer wavelengths where facility instruments are available. I will report on polarimetric results from our 2013 campaign, which demonstrate a sharp increase in the linear polarization fraction and variability of Sgr A* with increasing baseline. These data allow rich model-independent inferences about the polarization images and morphology and reveal that polarization is a sensitive probe of intrinsic variability.

Michael Johnson, Harvard-Smithsonian Center for Astrophysics

Deblurring Sgr A* Images

Scattering in the tenuous interstellar plasma blurs the image of Sgr A*. This effect decreases steeply with increasing frequency and becomes subdominant to the intrinsic emission structure at wavelengths close to a millimeter. I will discuss recent work that demonstrates how we can invert the blurring when properties of the scattering are known. With this technique, we can reconstruct the unscattered image of Sgr A* using EHT data. I will also show why some EHT observables -- such as closure phase and fractional polarization -- are largely immune to scattering. Finally, despite decades of study, there has been a recent flurry of progress in understanding the scattering properties, including studies of the Galactic Center magnetar and the discovery of refractive substructure in the scattering disk of Sgr A* at 1.3-cm wavelength. I will discuss these recent findings and their implications for imaging Sgr A* with the EHT

Serguei Komssarov, University of Leeds

Blandford-Znajek Mechanism: Event Horizon or Ergoregion?

There are good reasons to believe that relativistic jets of AGN are powered by rotating black holes via the Blandford-Znajek mechanism. Although the original mathematical solution, which demonstrated the possibility of such energy extraction, was found 37 years ago, its physical nature still remains a subject of debate.

I will give a brief review of some recent developments in this area with focus on the roles played by the EH and the Ergoregion in the Blandford-Znajek mechanism.

Michael Kramer, Max Planck Institute for Radio Astronomy

Testing gravity in the centre of the Galaxy

The supermassive black hole in the centre of the Milky Way, Sgr A*, is an ideal target for testing the properties of black holes. A number of experiments are being prepared or conducted, such as the monitoring of stellar orbits, the search for radio pulsars or the recording of an image of the shadow of a event horizon. The talk puts these efforts in context with other tests of general relativity and its alternatives. I will also compare the approaches in the Galactic centre, while concentrating in particular on the prospects of using pulsars as probes, also in combination with event horizon imaging.

Thomas Krichbaum, Max Planck Institute for Radio Astronomy

New Results from Global Millimeter VLBI observations - How small an AGN can be?

VLBI observations at the highest possible frequency penetrate the opacity barrier in the nuclear regions of radio-galaxies and blazars, which are synchrotron self-absorbed at longer wavelength. This facilitates a direct and sharper than ever view into the 'heart' of Active Galactic Nuclei (AGN), into region in which BH physics and general relativity effects become important and where radio jets are launched. Here we report on new results from global 3mm and 1.3mm VLBI observations adding the APEX and IRAM to the Event Horizon Telescope. New images and core size estimates for a number of AGN jets and for Sgr A* are presented and discussed.

Sgr A* is one of the few black holes whose capture radius is just resolvable with the <0.5” spatial resolution of the Chandra X-ray Observatory. Motivated by this potential we proposed for the deepest ever view of our Galactic center in 2012, via a Chandra-HETGS “X-ray Visionary Project”. Involving over 60 members of the Galactic center community, we obtained ~35 days of observations within a single year, for the first time also using the spectral gratings. This campaign doubled the photon count compared to the 12 years of observations before, and involved simultaneous observations from radio through gamma-ray frequencies. We are still analyzing these data sets, in combination with ongoing monitoring campaigns since, which together provide key constraints for the modeling of Sgr A* and environs necessary for the Event Horizon Telescope. I will present the highlights of this campaign so far, with a focus on results that can alter/define our ideas about the accretion geometry and plasma conditions near the black hole.

Dan Marrone, Univrsity of Arizona

Implementation of EHT Technical Developments

The path from the current EHT to the full array capabilities has been mapped out over the past several years. Through the efforts of the EHT Technical Working Group and the full complement of experts involved in the project, we have a detailed plan and work breakdown to implement these improvements. I will describe the schedule for EHT expansion and enhancement from 2015 onward.

Ivan Marti-Vidal, Onsala Space Observatory

Phased ALMA and high-fidelity polarimetry with the EHT

Circular polarization feeds are preferred for a robust calibration of VLBI observations. Indeed, most VLBI stations record in a circular polarization basis. However, all wide-band receivers (including those of ALMA) observe in linear polarization basis. We will discuss on the procedure followed to solve the problem of linear feeds in the Phased-ALMA polarization conversion for VLBI. We will show how a high-fidelity polarimetry can be obtained with our approach, when the Phased-ALMA signal is combined with circular-polarization signals from other VLBI stations. We will also discuss on the possibility, advantages and disadvantages, of using the same polarimetry strategy on other future VLBI stations with linear feeds (to increase the achievable bandwidth in VLBI). We will also present test results with real data, obtained from a 3mm fringe test among two EVN stations. Preliminary results from real ALMA Phasing data may also be shown.

Samir Mathur, Ohio State University

Fuzzballs, Firewalls and all that

Some 40 years ago Hawking showed that if the black hole has a smooth horizon, then information will be lost when the black hole radiates. In string theory black holes appear to have a complete set of `hair'; these black hole states are called fuzzballs, and they radiate like normal bodies with no information loss. It was recently argued that structure at the horizon will necessarily feel like a `firewall' to an infalling observer. We will show that this need not be the case, since one can have `fuzzball complementarity' where an approximately smooth horizon appears as a `dual' description.

Jeff McClintock, Harvard Smithsonian Center for Astrophysics

Measuring the Spins of Black Holes

Both the continuum-fitting and Fe-line methods of measuring black hole spin will be discussed and compared, with attention to sources of systematic error. Both methods rely on estimating the inner radius of the black hole's accretion disk and identifying it with the radius of the ISCO. The Fe-line method is extremely important because of its dominant role in measuring the spins of supermassive black holes, which is problematic for the continuum-fitting method. Meantime, both methods are applicable to stellar-mass black holes, and we will discuss current efforts to cross-check the spins of individual black holes. Finally, a comprehensive summary of spin results for both stellar-mass and supermassive black holes will be presented.

Jonathan McKinney, University of Maryland

Polarized emission from Black Hole Accretion Disks and Jets

We discuss how polarized emission can place constraints on the properties of accretion flows and jets in the strong gravity regime for systems like SgrA* and M87 being observed by the Event Horizon Telescope.

John Monnier, University of Michigan

Optimized Image Reconstruction: Insights from Optical Interferometry

The radio community pioneered the use of closure phases to allow interferometric imaging even when fringe phases are compromised by atmospheric turbulence or unstable reference clocks. Eventually, better receivers and observing methods allowed phase referencing to provide direct measures of complex visibilities and eased the uncertainties using Fourier inversions required for imaging. At the same time, optical and infrared (O/IR) interferometers have been developed recently that can combine up to 6 telescopes simultaneously, at which point the inadequacies of classic imaging methods such as CLEAN were apparent. Here I report on dramatic progress within the O/IR interferometry to develop new image reconstruction techniques taking advantage of advances in ``compressed sensing'' theory and new approaches afforded by modern computing. Because the Event Horizon Telescope must rely on closure phases instead of direct Fourier phases, the new algorithms from the O/IR could be essential to extracting the most information from EHT observations and we demonstrate promising results using simulated EHT data.

Monika Moscibrodzka, Radboud University Nijmegen

Two-temperature disk + isothermal jet model for Sgr A* and M87

The super-massive black hole in the center of the Milky Way, Sgr~A*, displays a nearly flat radio spectrum which is typical for jets in Active Galactic Nuclei. Indeed, time-dependent, magnetized models of radiatively inefficient accretion flows, which are commonly used to explain emission of Sgr A* also often produce jet-like outflows. However, the emission from these models so far has failed to reproduce the flat radio spectrum. We show that current GRMHD simulations can naturally reproduce the flat spectrum, when using a two-temperature plasma in the disk and a constant electron temperature plasma in the jet. This assumption is consistent with current state-of-the art simulations, in which the electron temperature evolution is not explicitly modeled. The model images and spectra are consistent with the radio sizes and spectrum of Sgr~A*. The model can also reproduce the radio images of the jet base in M87.

We examine the structure and dynamics of the M87 jet based on both multi-frequency observations and MHD jet theories. Millimeter (mm) VLBI cores are considered as innermost jet emissions. Resolved parabolic streamline may suggest that the jet collimation maintains in five orders of magnitude in the distance starting from the vicinity of the supermassive black hole (SMBH), less than 10 r_s where the VLBI core at 1.3 mm is located. Observed sub-to-superluminal motions may indicate an MHD acceleration takes place from non-relativistic to relativistic regimes. We discuss that the M87 jet consists hybrid spine/sheath structure from either a spinning super massive black hole and or radiatively inefficient accretion flow. Future sub-mm VLBI imaging play an essential role in resolving the origin (i.e., formation mechanism) of the M87 jet as well as constraining the SMBH spin parameter. Based on our understanding of M87, we also discuss about the key science in other AGN jets with sub-mm VLBI experiments.

Gopal Narayanan, University of Massachusetts

Status of the Large Millimeter Telescope (LMT) and EHT Preparations

The Large Millimeter Telescope (LMT) is a 50m diameter telescope at an altitude of 4600 m in the country of Mexico. It is a joint project between the University of Massachusetts, Amherst and the Instituto Nacional de Astrofisica, Optica y Electronica (INAOE). At this time, the inner three rings of the telescope have been furnished with precision surface panels for an effective diameter of 32.5 m and effective surface rms better than 80 microns. With an active primary surface, the LMT can maintain this surface over a wide elevation range. While the outer rings will be procured and installed in the next 2 years, the telescope is into its third year of early science with two receivers, the Redshift Search Receiver (RSR) and AzTEC. I will provide a short description of the engineering and scientific status of the LMT. I will also summarize VLBI activities at the LMT to date, and present plans for a dual-polarization 1mm wavelength receiver for the LMT that will enable the telescope to participate in the EHT experiment by 2016.

Dimitrios Psaltis, Univrsity of Arizona

GPU-accelerated Ray Tracing and a Null Hypothesis Test of GR with the EHT

The Event Horizon Telescope will generate the first images of the black-hole shadows in Sgr A* and in M87. The observed photons will have originated in one of the strongest gravitational fields found in the Universe, encoding during their travel to the Earth the properties of the black-hole spacetimes. In this talk, I will discuss the prospect of performing a new null hypothesis test of GR with EHT observations of Sgr A* that does not depend on a prior knowledge of the properties of the accretion flow. I will address a a small number of outstanding questions related to the scattering screen towards the galactic center that need to be answered in order for this null hypothesis test to be performed. I will then use results from recent GPU-accelerated ray tracing calculations in conjunction with GRMHD simulations to argue that upcoming observations of Sgr A* with the EHT will be able to confirm the GR predictions for the size and shape of the black-hole shadow to an accuracy of better than 10%, in a model independent way.

I summarize several physical processes that can produce efficient particle acceleration in radiatively inefficient accretion flows. I then describe the implications for non-thermal emission and EHT observations of Sgr A*.

I will discuss the current status of the NANOGrav pulsar timing array, and the prospects for a detection of the stochastic background produced by the mergers of supermassive black holes.

Alexander Tchekhovskoy, University of California, Berkeley

Horizon Scale Lepton Acceleration in Jets: Explaining the Compact Radio Emission in M87

It has now become clear that the radio jet in the giant elliptical galaxy M87 must turn on very close to the black hole. This implies the efficient acceleration of leptons within the jet at scales much smaller than feasible by the typical dissipative events usually invoked to explain jet synchrotron emission. Here we show that the stagnation surface, the separatrix between material that falls back into the black hole and material that is accelerated outward forming the jet, is a natural site of pair formation and particle acceleration. This occurs via an inverse-Compton pair catastrophe driven by unscreened electric fields within the charge-starved region about the stagnation surface and substantially amplified by a post-gap cascade. For typical estimates of the jet properties in M87, we find excellent quantitive agreement between the predicted relativistic lepton densities and those required by recent high-frequency radio observations of M87.

Remo Tilanus, Radboud University Nijmegen

ETWG Report and System upgrades for Spring 2015 mmVLBI Campaign

Convened in the summer of 2013, the EHT Technical Working Group (ETWG) was tasked with:

1. Survey the capabilities at all EHT facilities

2. Establish a set of specifications for future EHT observations

3. Outline the technical developments needed to reach these goals

4. Based on prioritized science objectives, formulate a project roadmap that is grounded in technical feasibility with the resources available.

This overview will report on the findings and conclusions arrived at by the ETWG and present specifications for future EHT observations at 230 and 345 GHz. I will also briefly discuss subsequent technical developments and the rollout of new equipment for the 2015 EHT observing campaign that will for the first time deploy recording at 16 Gb/s. With a relatively modest expansion over the next couple of years this mmVLBI system will be capable of 64 Gb/s for an aggregate bandwidth of 16 GHz i.e. 8x the bandwidth of existing EHT observations.

Laura Vertatschitsch, Harvard Smithsonian Center for Astrophysics

VLBI Backend Systems

The EHT data acquisition campaign in March 2015 will utilize new digital backends on many of the new and existing sites. We will briefly overview the state of the art for wideband digital backends for VLBI and discuss the several flavors that will be employed this coming year. Specifically, we will focus on details of the Roach2 Digital Backend (R2DBE), a new 16 Gbps wideband DBE unit that can be scaled to 64 Gbps, to be employed at single dish sites. The R2DBE digitizes 2.048 GHz of RF in each of two inputs and sends 8 Gbps single-channel data on each of two 10 GbE links to a Mark 6 data recorder with expansion chassis for a total of 16 Gbps. The system utilizes open source hardware, firmware, and software developed through the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER). Engineers at SAO have been members and developers for the CASPER group for several years, thus the challenge of rapidly prototyping a VLBI backend system on a limited budget and short time scale could be met by drawing on years of previous development and expertise. In four months the system was developed and demonstrated in 32 Gbps form for the South Pole Telescope, achieving fringes in a hybrid correlation between an R2DBE and an existing R1DBE. We present the tests and results achieved with the R2DBE 32 Gbps system for the South Pole, and upcoming developments in preparation for March 2015.

Marta Volonteri, Paris Institute of Astrophysics

Growth of supermassive black holes and their relationships to their host galaxies

Black holes are the engines that power quasars and active galactic nuclei throughout cosmic time. The masses of black holes in nearby galaxies define clear correlations with the properties of their host galaxies. These results suggest that black holes, while a thousand times lighter than the galaxy, grow alongside their hosts during its cosmic evolution. I will discuss the growth of black holes, and the establishment of the connection between galaxies and black holes.

Jonelle Walsh, Texas A&M University

Gas Dynamical Black Hole Mass Measurements for M87

M87 is one of the most luminous nearby galaxies and hosts one of the most massive black holes known, making it a very important target for extragalactic studies. The supermassive black hole has been the subject of several stellar and gas dynamical mass measurements; however, the best current stellar dynamical black hole mass is larger than the gas dynamical determination by a factor of two, corresponding to a 2-sigma discrepancy. In this talk, I will review the gas dynamical black hole mass measurements that have been made over the years for M87, focusing in particular on the most recent measurement from multi-slit Space Telescope Imaging Spectrograph observations from the Hubble Space Telescope. I will also discuss the strengths and weaknesses generally associated with stellar and gas dynamical black hole mass measurement methods, and the current state of cross-checks between the two methods that have been carried out within the same galaxy.

Jonathan Weintroub, Harvard Smithsonian Center for Astrophysics

Wideband digital signal processing for EHT & SMA

To improve the EHT’s sensitivity, resolution and imaging fidelity one might enhance station collecting area, add stations, or increase bandwidth. The latter improvement, bandwidth, is probably the most accessible. For many stations, including Submillimeter Array (SMA), the bandwidth is limited not by the receiver but by the digital signal processing. This talk will describe digital development at SMA. The upgraded SMA will contribute directly to the EHT, and technology under development has potential to benefit other EHT stations as well.

Christiann Brinkerink, Radboud University Nijmegen

What closure phase measurements at 86 GHz can tell us about Sagittarius A*

Thomas Bronzwaer, Radboud University Nijmegen

Predicted VLBI Observables for SMBH's Based on GRMHD Simulations

Alejandro Cardenas-Avendano, Universidad Nacional de Colombia

Probing the Regular Nature of the Spacetime by Measuring the Shadow of a Black Hole

Andrew Chael, Harvard-Smithsonian Center for Astrophysics

Maximum Entropy Polarimetric Imaging

Laura Chajet, York University

AGNs: Line Width Distribution of MHD Disc Winds

Pierre Christian, Harvard Smithsonian Center for Astrophysics

complimentary observations to the EHT for understanding strong gravity

To improve the EHT?s sensitivity, resolution and imaging fidelity one might enhance station collecting area, add stations, or increase bandwidth. The latter improvement, bandwidth, is probably the most accessible. For many stations, including Submillimeter Array (SMA), the bandwidth is limited not by the receiver but by the digital signal processing. This talk will describe digital development at SMA. The upgraded SMA will contribute directly to the EHT, and technology under development has potential to benefit other EHT stations as well.

The EHT Wiki is the primary vehicle for communication with the project. The site contains pages on science investigations, algorithmic development, new hardware, and staging information for observations and data processing. This talk will introduce the Wiki and walk through its various sections to show how it is used to help organize the EHT.

You've just finished running your code, and you're certain that you (and only you) know exactly what the region around a supermassive black hole looks like. You could lie back and wait for the accolades to roll in, but why not take an extra moment to make testable predictions that even the observers can understand? Synthetic data can help.

Scattering in the tenuous interstellar plasma blurs the image of Sgr A*. This effect decreases steeply with increasing frequency and becomes subdominant to the intrinsic emission structure at wavelengths close to a millimeter. I will discuss recent work that demonstrates how we can invert the blurring when properties of the scattering are known. With this technique, we can reconstruct the unscattered image of Sgr A* using EHT data. I will also show why some EHT observables -- such as closure phase and fractional polarization -- are largely immune to scattering.

The angular resolution is the most fundamental to imaging the event-horizon-scale structure of supermassive black holes, and in order to realize the highest angular resolution ever, the EHT has been developing a world-wide (sub)mm VLBI array under the international collaborations.In order to boost the imaging capability of the EHT, we have been developing a new imaging method based on the technique so-called "sparse modeling", which allows us to directly solve the ill-posed Fourier-transform equations caused by incomplete sampling of visibilities.We show that the images obtained based on su

Sgr A* regularly flares in the X-ray and near-IR on ~hour timescales, and the EHT has already detected interday variability in 1.3 mm emission on long and short baselines. The addition of highly sensitive long baselines in 2015 will allow for the resolution of time variable structure on sub-minute timescales. This opportunity to observe dynamical process on event horizon scales comes with the challenge of sparse visibility coverage, but several strategies can recover rich information from the limited samples.

Maximizing the science return on the Event Horizon Telescope project requires fitting models for spatially resolved black hole images to the data. These images can be calculated from accretion and jet theory, but theoretical uncertainties lead to systematic errors in the predicted images. In many cases, however, the images are dominated by the combined effects of Doppler beaming and light bending, leading to a characteristic “crescent” shape. I will discuss a geometric crescent model for black hole images based on these effects.

We are now two years into development of hardware and software for phasing ALMA, with emphasis on Band 6 (1.2 mm) but applicability to bands 3 and 7. Central to the effort is software to continuously calculate and apply a phasing solution, and the hardware interface cards (PIC) and other upgrades to the correlator that apply the solution, format the data and extract the correlated data stream. Upstream from the correlator is a hydrogen maser, which is the new ALMA time standard.

Over the next few years the Event Horizon Telescope will greatly expand its capabilities from the current 8 Gbps at a few sites to 64 Gbps at a much larger number of sites. The good news is that this processing can proceed through 4 independent (16 Gbps) processing stages of 2 GHz of bandwidth. The bad news is that EHT stations come in multiple "flavors" each posing its own issues for correlation with DiFX (the current correlation option). This talk will discuss some of the issues and the road ahead.